Recent developments of medical technologies include noninvasive imaging techniques of tracer molecule's distribution in vivo using tomography. For example, positron emission tomography (PET) devices are utilized as one of diagnostic apparatuses in the field of nuclear medicine. In PET devices, detection is made for two pencils of gamma rays, or hereinafter “pair-annihilation gamma rays,” which are emitted when a positron emitted through beta decay (β+ decay, or positron decay) makes pair-annihilation with an electron in surrounding media. The pair-annihilation gamma rays are a pair of gamma rays that are emitted into opposite directions of substantially 180 degrees therebetween, each of which has energy of 511 keV. In the PET devices, a number of straight lines are identified, each of which connecting a pair of detectors that detected the pair-annihilation gamma rays, and thereby a distribution is estimated for a nuclide that make transitions with the beta decay. For example, molecules of a medical agent that accumulate in cancer cells are labeled with a positron emitting nuclide and imaging the living body, or the subject to be imaged, to which the medical agent has been administered, by the PET device; then a three dimensional in vivo distribution image of the cancer cells is obtained. Compared to a single photon emission computed tomography (SPECT), which is also a diagnostic imaging device in nuclear medicine like the PET device for imaging in vivo functional images of the living body, the PET device is generally superior in sensitivity and qualitative performance, because PET devices do not require any collimator for gamma rays.
On the other hand, according to the developments in life science or biomedical science it has been revealed that complex interrelated dynamics among a plurality of molecules is actually underlying the activities of living organisms, and would be related to initiation of lesions. What has been studied in anticipation of such applications is administering multiple medicines labeled with different radioactive nuclides to obtain distribution images of respective medicines at a time (“simultaneous imaging on multi-tracer”). To realize the simultaneous imaging on multi-tracer, such techniques as SPECT and use of Compton cameras are adopted.
On the other hand, a technique is disclosed for detecting gamma rays emitted in a gamma decay that takes place following a positron decay, where energy of each gamma ray is specific to nuclide, or “a unique gamma ray,” and where the detection is made together with the pair-annihilation gamma rays of the positron decay. For example, Non-Patent Document 1 (James D. Kurfess et al, IEEE Nuclear Science Symposium Conference Record, 2001 vol. 2 p. 1166-1170) and Patent Document 1 (U.S. Pat. No. 4,833,327) disclose improving resolution of PET images by using unique gamma rays from a single nuclide.